The present invention relates to a low-drop voltage regulator.
A voltage regulator is an electronic device primarily for supplying current to a load connected to its output, while maintaining the voltage at the output as constant as possible. For this purpose, the device presents an input pin from which it draws the current to the load plus its own operating current.
Voltage regulators are of two types: a first provides for maintaining a higher voltage at the output than at the input, and as such requires reactive elements (transformers, reactors) which represent a drawback in terms of cost, weight and size; the second operates conversely, and comprises capacitive elements, which are more advantageous in terms of the above drawbacks.
Capacitive regulators, to which the present invention relates, feature a storage condenser and a diode between the input and the condenser, which enable them to operate perfectly even if the input voltage is less than the output voltage, or indeed zeroed or inverted, for a limited length of time.
Voltage regulators must provide for:
a) maintaining a constant output voltage despite even rapid variations in supply voltage at the input or in output current; and PA1 b) reliable, long-term performance, particularly in the case of automotive applications in which failure of the regulator may result in drastic consequences. PA1 1) supplying the load with rated service voltage and current, even when the input voltage is extremely close to the rated load voltage (in particular, when the difference between the two voltages is roughly 0.5 V); PA1 2) maintaining the rated voltage and current at the load, even in the event of a temporary interruption in supply, i.e. an input voltage equal to or less than the rated output voltage, including negative values; PA1 3) maintaining operation as per point 2) for as long as possible. This depends on the amount of energy stored in the condenser (which in turn depends on the rated supply voltage and the size of the condenser) and the rate at which it is drawn off (which depends on the output current and the current consumed by the regulator itself).
In particular, the regulator dealt with herein must provide for:
A known voltage regulator featuring the above characteristics is described, for example, in EP-A-110.775 and illustrated by way of reference in FIG. 4. This device, numbered 10 in FIG. 4, presents an input terminal 11 connectable to a voltage source 12 (supplying input voltage V.sub.A) and an output terminal 13 connectable to a load 14. Input terminal 11 is connected to the emitter of a first PNP power transistor 16, the collector of which is connected to output terminal 13, and the base of which is connected to the output of an operational amplifier 17 with a current output. Input terminal 11 is also connected to the anode of a diode 18, the cathode of which is connected to the emitter of a second PNP power transistor 19 also having its collector connected to output terminal 13 and its base connected to the output of amplifier 17. A condenser 21 is connected between the cathode of diode 18 and ground; and operational amplifier 17 presents its non-inverting input connected to output terminal 13, and its inverting input connected to a voltage source 22 supplying reference voltage V.sub.R.
In the known circuit in FIG. 4, the emitter of transistor 16 is normally biased to a higher voltage than that of transistor 19, on account of the voltage drop V.sub.D in diode 18, whereas the two bases present the same potential. Consequently, transistor 19 is off, and load 14 is supplied with current by transistor 16.
Operational amplifier 17 ensures voltage V.sub.o of load 14 remains equal to reference voltage V.sub.R. When output voltage V.sub.o falls below the above value, operational amplifier 17 draws off additional current from the base of transistor 16, so as to increase current supply from the collector of transistor 16 to the load, until output voltage V.sub.o is again equal to reference voltage V.sub.R.
The above known circuit operates as described even in the event of input voltage V.sub.A falling to such an extent as to practically equal output voltage V.sub.o. Transistor 16 in fact is capable of transferring current from the emitter to the collector until the difference in potential between the two equals the saturation voltage of transistor 16 (usually below 0.5 V).
Moreover, when operating as described above, in the event of input voltage V.sub.A falling suddenly, or even being zeroed or inverted, so that the voltage at the emitter of transistor 19 (equal to that of condenser 21) exceeds the voltage at the emitter of transistor 16 (equal to input voltage V.sub.A), transistor 16 goes off, and transistor 19 comes on to supply the load.
Operation as described above is possible as long as the voltage of condenser 21 is higher than the rated output voltage (equal to V.sub.R), i.e. as long as the voltage of the condenser equals the rated output voltage plus the saturation voltage of transistor 19 between the emitter and collector, as in the case of transistor 16. This operating mode is sustainable for a time interval T.sub.M equal to the time taken by condenser 21 to discharge. That is, if V.sub.sat2 is the saturation voltage of transistor 19; I.sub.L the load current; and I.sub.B the current required for the circuit to function, said time interval equals: ##EQU1## With the following typical values: V.sub.A =14 V; V.sub.D =0.7 V; V.sub.R =5 V; V.sub.sat2 =0.5 V; I.sub.C =300 mA; I.sub.B =10 mA; and C=220 .mu.F; this gives a time interval T.sub.M of 5.6 ms, after which it is no longer possible to supply the load. The above known circuit presents a number of drawbacks. Firstly, PNP transistors are large, high-cost components, each of which must be sized to withstand maximum output current and severe electric and thermal stress. This is especially critical when the regulator is employed under extreme operating conditions involving considerable instantaneous power dissipation, such as shortcircuiting at the output with a sharp rise in current; or high overvoltage at the input, as in the case of automotive applications; or when a strong overcurrent is requested during startup for bringing the output voltage V.sub.o up to the operating value in an extremely short space of time. All these situations impair the reliability and, in particular, the working life of the circuit.
Another drawback of the above known circuit lies in the switching of transistors 16 and 19. When switching from one to the other, it is highly probable that, for an albeit brief period of time, neither is operative, due to the slow startup capability of large-size PNP transistors, thus resulting in a brief interruption in current supply to the load. Moreover, switching occurs fairly frequently. Even a brief transient state capable of rapidly reducing input voltage V.sub.A by slightly more than V.sub.D (e.g. 1 V) and the emitter voltage of transistor 16 to below that of transistor 19 is sufficient for switching the two transistors and so causing interference at the output. Further interference is caused when the input voltage is restored or the condenser discharged, thus enabling transistor 16 and disabling transistor 19.